Chromium white irons, in particular high chromium white irons, resist wear as a result of their content of very hard M7C3 carbides, where M is Fe,Cr or Cr,Fe but may include small amounts of other elements such as Mn or Ni, depending upon the composition. The wear resistant high chromium white irons may be hypoeutectic, eutectic or hypereutectic.
The hypoeutectic chromium white irons have up to about 3.0% carbon, and their microstructure contains primary dendrites of austenite in a matrix of a eutectic mixture of M7C3 carbides and austenite. The eutectic white irons have from about 3.0% to about 4.0% carbon and a microstructure of a eutectic mixture of M7C3 carbides and austenite. The hypereutectic chromium white irons have from about 3.5% to about 5.0% carbon, while their microstructure contains primary M7C3 carbides in a matrix of a eutectic mixture of M7C3 carbides and austenite. In each case, it is the presence of the M7C3 carbides, either as eutectic carbides or primary carbides, that provides the alloy with its wear characteristics. The hypereutectic white irons are considered to have higher volume fractions of the hard and wear resistant M7C3 carbides than the hypoeutectic white irons, and are thus often the preferred alloy for many hardfacing applications. However, the hypereutectic white irons generally are not favoured for casting, due to stress induced cracking during cooling.
It is widely recognised in the art that, with the increase in wear resistant properties available with hypereutectic high chromium white irons, there is a corresponding decrease in fracture toughness. High chromium white cast irons are used extensively in mining and mineral processing industries, in applications in which their abrasion resistance is required, but in which relatively low fracture toughness is acceptable. However, there are other applications where low fracture toughness has not been acceptable. This has meant that hypereutectic high chromium white cast irons have not been usable and there have been various attempts to address this.
The background section of Australian patent application AU-A-28865/84, which primarily relates to high chromium white cast irons of both hypoeutectic and hypereutectic compositions, describes the many failed attempts to develop satisfactory hypereutectic white iron alloys for castings, which combine wear resistance with fracture toughness. AU-A-28865/84 also describes various attempts to develop hypoeutectic compositions, and draws on attempts in the art to develop suitable hardfacing alloys as providing possible solutions to the wear resistance vs fracture toughness dilemma. However, AU-A-28865/84 in fact predominantly solves the cracking problem of cast compositions by forming them as cast composites—namely by creating a composite component comprising the preferred alloy metallurgically bonded to a substrate, thus assisting with avoiding the likelihood of cracking upon the cast alloy cooling. Indeed, AU-A-28865/84 seeks to overcome the disadvantages of low fracture toughness and cracking with hypereutectic castings having greater than 4.0 wt. % carbon by ensuring the formation in a composite casting of primary M7C3 carbides with mean cross-sectional dimensions no greater than 75 micron, and suggests a variety of mechanisms for doing so. Thus, AU-A-28865/84 aims to overcome the problem by forming composite components and limiting the size of the primary M7C3 carbides in the alloy itself.
U.S. Pat. No. 5,803,152 also seeks to refine the microstructure of, in particular, thick section hypereutectic white iron castings, in order to maximise nucleation of primary carbides, thereby enabling an increase not only in fracture toughness but also in wear resistance. This refinement is achieved by introducing a particulate material into a stream of molten metal as the metal is being poured for a casting operation. The particulate material is to extract heat from, and to undercool, the molten metal into the primary phase solidification range between the liquidus and solidus temperatures.
In relation to previous attempts to improve fracture toughness in hardfacing alloys, U.S. Pat. No. 6,375,895 points out that most prior art high chromium white irons for hardfacing always show a more or less dense network of cracks (or check cracking) in the as-welded condition, despite precautions to avoid this. U.S. Pat. No. 6,375,895 indicates that the comparative hardness of primary carbides (about 1700 Brinell hardness number (BHN)) in a soft austenite matrix (about 300 BHN to 600 BHN) gives rise to shrinkage cracks on cooling from the molten state. The solution offered by U.S. Pat. No. 6,375,895 is to adopt a particular alloy composition, pre-heating of the base component to be hard-faced, and subsequent cooling regimes, that ensure a substantial martensitic presence in the microstructure and a consistent hardness (about 455 BHN to 512 BHN) throughout the alloy.
It is an aim of the present invention to provide a wear resistant, high chromium white iron that is able to be cast or used as a hardfacing alloy substantially crack free. When used to produce castings, the white iron of the invention does not require the formation of composite components, or the use of complex casting techniques. Also, the use of costly pre-heating techniques are not necessary for use of the white iron for hardfacing.
Before turning to a summary of the invention, it is to be appreciated that the previous description of prior art is provided only for background purposes. Reference to this prior art is not to be considered as an acknowledgement that the disclosure of any of the documents considered is well known or has entered the realm of common general knowledge in Australia or elsewhere.